1. Field of the Invention
The present invention relates to a microcapsule that is used in recording materials such as heat-sensitive recording materials and pressure-sensitive recording materials, and, more particularly, to a microcapsule that makes it possible to freely design the characteristics of a capsule such as storage stability, light resistance or humidity dependence and can be manufactured according to a simple and easy manufacturing method.
2. Description of the Related Art
As methods of forming microcapsules, there are various conventionally known methods such as a method that makes use of coacervation of a hydrophilic wall forming material (see, for example, U.S. Pat. No. 2,800,457); an interfacial polymerization method (see, for example, U.S. Pat. No. 3,287,154, U.K. Patent No. 990443, and Japanese Patent Application Publication (JP-B) No. 38-19574); a method using polymer precipitation (see, for example, U.S. Pat. No. 3,418,250); a method that uses an isocyanate polyol wall material (see, for example, U.S. Pat. No. 3,796,669); a method that uses an isocyanate wall material (see, for example, U.S. Pat. No. 3,914,511); a method that uses urea-formaldehyde-based and urea formaldehyde-resorcinol-based wall forming materials (see, for example, U.S. Pat. No. 4,001,140); a method that uses wall forming materials such as melamine-formaldehyde resins, hydroxypropyl cellulose and so on (see, for example, U.S. Pat. No. 4,025,455); an in situ method due to monomer polymerization (see, for example, JP-B No. 36-9168); an electrolytic dispersion cooling method (see, for example, U.K. Patent No. 952807); and a spray drying method (see, for example, U.S. Pat. No. 3,111,407).
Specifically, in the interfacial polymerization method, an oil phase in which a core-forming material is dissolved or dispersed in a hydrophobic organic solvent is mixed with a aqueous phase that contains a water-soluble polymer followed by emulsifying and dispersing using a homogenizer and further followed by heating, whereby a polymer forming reaction is caused at an oil/water interface. As a result, a microcapsule wall made of a polymer is formed and encapsulation is attained. In view of the advantages that the method can provide microcapsules excellent in storage stability and having a uniform particle diameter in a short time, the interfacial polymerization method is in wide use.
On the other hand, in the manufacture of microcapsules, it is necessary to control the characteristics of a capsule in accordance with the field in which it is used.
As a technology that can improve the resistance of microcapsule against moisture and humidity, see, for example, Japanese Patent Application Laid-Open (JP-A) No. 5-7767 proposes a double-wall microcapsule including a primary wall made of an amino resin and a secondary wall made of a poly-ion complex of a cationic polyamide-epihalohydrin resin and polystyrene sulfonic acid. However, there has been a problem in that the range of selection of desired characteristics is quite limited. Accordingly, a microcapsule manufacturing method that can simply and easily control various characteristics has been required.
Furthermore, JP-A No. 6-339624 proposes a heat-resistant microcapsule in which a wall film is made of a particular melamine resin and a heat resistant layer made of heat resistant particles and silicone oil is coated on a surface of the wall film. However, in the method of generating such heat-resistant microcapsules, there is a problem in that adjustment under a particular environment is necessary. Accordingly, there is a demand for development of a more versatile method.
Furthermore, in a case where coating is applied to the microcapsule as mentioned above, it is also important to improve a covering state thereof.